In 2009, we discovered a new family of enzymes, defined by the presence of a 'Fic' (filamentation induced by c-AMP) domain, that post-translationally adenylylate their protein substrates to alter cellular signaling. Fic proteins are conserved from bacteria to humans. In addition to catalyzing the transfer of an adenosine- monophosphate (AMP) moiety from ATP to their target proteins, they can also catalyze the transfer of phosphorylcholine, uridine-monophosphate (UMP), guanidine-monophosphate (GMP), and phosphates. Post- translational modifications (PTMs) catalyzed by Fic proteins are now recognized as a universal signaling paradigm that regulates critical processes like bacterial pathogenesis, eukaryotic signaling, drug tolerance, and bacterial stress. Although these discoveries have greatly expanded our knowledge of the biochemical capacity of these enzymes, our understanding of the >8000 Fic-family members is still rudimentary. Our goal is to understand the role of Fic-mediated adenylylation as a PTM in eukaryotic signal transduction. We previously reported that the Fic domain(s) of the protein IbpA from the pathogenic bacterium Histophilus somni adenylylates and inhibits mammalian RhoGTPases, RhoA, Rac1 and Cdc42. This event induces host cytoskeletal collapse, allowing H. somni to breach alveolar barriers. The adenylylation occurs on a functionally critical switch 1 Tyrosine of GTPases. We also successfully crystallized the IbpA(Fic domain)-Cdc42-ATP complex. This co-crystal provided incredible insight into the mechanism of adenylylation and substrate recognition/specificity. Thus, additional structure-function studies, particularly for those Fic proteins with substrates other than RhoGTPases, are warranted. Little is known about eukaryotic Fic proteins. The human genome encodes a single Fic protein, called HYPE. We showed that HYPE also functions as an adenylyltransferase but its target had remained elusive. Additionally, Drosophila HYPE, a close homolog of human HYPE, has been implicated in blindness by an unknown mechanism. We, therefore, seek to investigate the functional repertoire of the Fic family of enzymes, focusing on the unique human Fic protein HYPE. Here, we demonstrate that the endoplasmic reticulum chaperone, BiP, is adenylylated by HYPE. BiP monitors protein misfolding and maintains ER homeostasis by activating the unfolded protein response (UPR) in cells. We show that HYPE expression is upregulated upon UPR stress. We aim to 1) elucidate the consequences of BiP adenylylation during UPR progression; 2) understand the structural basis of the HYPE-BiP interaction; and 3) identify and characterize other targets of HYPE-mediated adenylylation. We predict that HYPE-mediated adenylylation of BiP will be an important new aspect of ER homeostasis regulation. Our structural analyses will provide clues for catalysis and substrate recognition. It is a truism that nature uses similar mechanisms over and over. Signaling networks similar to HYPE-mediated regulatory adenylylation are likely to occur in other organisms and may be targets for pathogenic Fic proteins.